In the manufacture of tissue products such as bath tissue, a wide variety of product characteristics must be given attention in order to provide a final product with the appropriate blend of attributes suitable for the product's intended purposes. Improving the softness of tissues is a continuing objective in tissue manufacture, especially for premium products. Softness, however, is a perceived property of tissues comprising many factors including thickness, smoothness, and fuzziness.
Traditionally, tissue products have been made using a wet-pressing process in which a significant amount of water is removed from a wet-laid web by pressing the web prior to final drying. In one embodiment, for instance, while supported by an absorbent papermaking felt, the web is squeezed between the felt and the surface of a rotating heated cylinder (Yankee dryer) using a pressure roll as the web is transferred to the surface of the Yankee dryer for final drying. The dried web is thereafter dislodged from the Yankee dryer with a doctor blade (creping), which serves to partially debond the dried web by breaking many of the bonds previously formed during the wet-pressing stages of the process. Creping generally improves the softness of the web, albeit at the expense of a loss in strength.
Recently, throughdrying has increased in popularity as a means of drying tissue webs. Throughdrying provides a relatively noncompressive method of removing water from the web by passing hot air through the web until it is dry. More specifically, a wet-laid web is transferred from the forming fabric to a coarse, highly permeable throughdrying fabric and retained on the throughdrying fabric until it is at least almost completely dry. The resulting dried web is softer and bulkier than a wet-pressed sheet because fewer papermaking bonds are formed and because the web is less dense. Squeezing water from the wet web is eliminated, although subsequent transfer of the web to a Yankee dryer for creping is still often used to final dry and/or soften the resulting tissue.
Even more recently, significant advances have been made in high bulk sheets as disclosed in U.S. Pat. Nos. 5,607,551; 5,772,845; 5,656,132; 5,932,068; and 6,171,442, which are all incorporated herein by reference. These patents disclose soft throughdried tissues made without the use of a Yankee dryer. The typical Yankee functions of building machine direction and cross-machine direction stretch are replaced by a wet-end rush transfer and the throughdrying fabric design, respectively.
Although the above-identified U.S. patents have provided great advances in the art, further improvements are still desired, especially with respect to increasing bulk and absorbency of the products without compromising strength. For example, in order to achieve higher bulk and absorbency in multi-ply tissue products, in the past, the individual plies have been embossed prior to bonding the plies together. Unfortunately, however, embossing the tissue webs may degrade the strength of the overall product. The embossing process also requires an additional process step that tends to increase the overall cost of the product and lower the rate at which the product is made.
In view of the above, a need currently exists for an improved multi-ply tissue product with enhanced bulk and absorbency characteristics.